Queensland Drought 2018, reactive soil, clay soil
Building & Renovating

Brisbane’s Clay Soil Hotspots – Is Your House at Risk?

One of the biggest threats to your Brisbane property is lurking right under your feet. Reactive soil has the potential to warp and crack your walls, damage your pipes and drainage systems, crack your pool and wreak havoc on the appearance and structural integrity of your home. Worse still, the damage caused by reactive soil can cost a fortune to repair – and it might not be covered by your insurance.

No home in Brisbane is safe from the threat of reactive soil. Here’s our go-to guide on identifying whether you have reactive soil and what you can do about it if you’ve got it.

Reactive soil

Reactive soils are soils that are prone to expanding when wet and shrinking when dry. Most reactive soils are clay soils. Unless your home is built on very sandy soil or on a rock base, there’s a strong possibility that it’s built on reactive soil. There are, however, degrees of reactivity. Building sites are classified into 6 separate categories ranging from non-reactive through to extremely reactive and problematic sites.

It’s important to accurately identify which soil type you have. That’s because the extent to which you have reactive soil affects several aspects of your build, including:

  • How you can use your land
  • The type of structure you can build
  • The design and methods you can use in constructing it.

It’s not just relevant to new builds. Reactive soil is an equally important consideration if you own or are buying an existing property, extending your home or installing a pool.

Where you’d find reactive soil in Brisbane – and why we’re seeing more of it

No doubt you’ve noticed the new housing estates popping up all over Brisbane, particularly on the urban fringes. Demand for affordable housing has meant that Brisbane’s become a property hotspot. Rural land is being snapped up at unprecedented speed and subdivided into housing estates. Unfortunately, that’s part of the problem.

Cycles of flooding, drought and erosion by strong winds over time can lead to exposure of underlying clay pans and result in reactive soil. That’s because the topsoil’s been stripped back or lost completely. Developers are now considering land which may have previously been written off as unsuitable for housing development due to reactive soil. While there are some innovative building techniques that cater for reactive soils, building is a cut-throat industry. Shortcuts are made to save money and make a property more affordable and attractive to you, the buyer. Measures that should have been used to combat the effects of reactive soil can be one of the first casualties of cost-cutting.

It’s not just the new estates that are plagued by reactive soil though: older sites in Brisbane have escaped rigorous soil testing. That’s because they were developed prior to soil testing becoming established practice. Just because you’re considering building or purchasing an existing property in an older pocket of Brisbane doesn’t mean that you can escape the potential pitfalls of reactive soil.

Reactive soil – why it’s bad news for your Brisbane property

So, why the big fuss about reactive soil? It’s because reactive soil causes ‘heave’. It’s a type of movement in the soil caused by expansion of the soil once it becomes wet. Think of a raindrop or a stone hitting the surface of a pond: heave is a bit like the shockwaves that cause the leaves or debris on the surface of the water to bob up and down.

In reactive soil that’s been saturated, the soil expands both horizontally and vertically. In other words, it moves outwards and upwards, exerting great pressure on everything with which it comes into contact. It’s powerful enough to lift concrete. The pressures in the soil will be transferred into the foundations or slab and then into the walls and ceilings of the property, causing cracking. This is called ‘stress transfer’. Unfortunately it doesn’t stop there. When the soil dries out again, it shrinks and drops, undermining any structure built on top of it.

Where the damage caused by reactive soil is extensive and has been allowed progress unabated, it can be alarming. Signs include unsightly cracks in your walls and ceilings, uneven floors and gaps in the external brickwork of your property. Reactive soil can even leave your home completely uninhabitable.

Reactive soil can have a devastating impact on your hip pocket. It can cause extensive damage, resulting in costly major repairs. Demolition may be necessary in extreme cases.

Reactive soil – The damage you can’t see

The most worrying aspect of reactive soil is that it can create damage that you can’t always see. It’s not just your walls and ceilings that can suffer. There are other parts of your home that can also bear the brunt of its effects.

Your slab and footings

Reactive soil can weaken or even crack your slab or footings. Repairing a cracked slab or cracked footings or foundations can be incredibly difficult and costly. Cracks such as those you see from reactive soil also provide an easy access point to your home for subterranean termites.

Pipes and drainage

Reactive soil can also damage the complex network of water and sewerage pipes and drains servicing your property. Expansion and contraction of the soil as it goes through a cycle of getting wet and drying out can shear the joins in these pipes and drains. In turn this causes leaks or even complete rupture. A leaking or broken pipe can further exacerbate the problems caused by reactive soil. That’s because it will saturate the soil and cause further expansion.

A broken or leaking pipe or drain is a huge problem itself. It can cause wood to rot and cause damp, mould and mildew. Mould and mildew can aggravate existing health problems such as asthma and respiratory problems in your family members, especially young children and those with compromised immune systems. Damage to a sewerage pipe comes with its own set of health risks and unpleasant consequences. Not only that, but any kind of leakage of water will attract termites into your property, as they thrive in a warm, humid environment.

If you’re building your home on reactive soil you need to plan ahead to factor in enough room for movement around your pipes and drains. Your fittings such as joins in pipes and connections will need to be engineered specifically to be flexible, to expand or even swivel so as to accommodate soil movement.

Your pool

If you’ve already got, or are planning on building a pool, the topic of reactive soil should have you sitting up and paying attention. Given it’s propensity to expand and shrink with enough force to crack concrete, reactive soil can be an unmitigated disaster if you’re a pool owner.

Reactive soil – why you might miss the crucial warning signs

If you’re in the market to buy a property, it’s easy to miss the tell-tale signs of damage caused by reactive soils. Cracks in internal walls can be hastily repaired and painted over. Render on the exterior of a property can conceal external cracking, as can strategic planting of screening bushes or shrubs and creepers. A pool can be filled up. Because a cycle of wetting and drying of the soil can take place over a period of years, the symptoms can be treated but the underlying cause might not be recognised. You don’t really stand a chance when it comes to picking up on the signs of damage caused by reactive soil, especially when a vendor is hell-bent on hiding the evidence.

Protecting yourself from the devastating impact of reactive soil

So, how can you protect yourself when you’re making what could be the biggest purchase of your life? There are two main ways:

  1. Insisting on soil testing
  2. Getting a comprehensive and professional property and building inspection done.

1. Testing for reactive soil

The only way to ascertain whether soil is reactive or not is to get a professional soil test done. This is a particularly crucial stage in the planning, approval and construction of new homes, pools and extensions.

Any soil identification and testing that you have carried out in relation to your property should strictly adhere to the appropriate Australian standard. It should also be carried out by a qualified geotechnical engineer who can furnish you with a comprehensive report.

2. Reactive soils and building inspections

One of the strongest safeguards against being stuck with a property that’s plagued by these problems is to organise a comprehensive and professional property and building inspection before you buy. It’s the only way to ensure that you’re not getting the wool pulled over your eyes when it comes to reactive soil.

It’s not always possible to avoid buying a home that’s been built on reactive soil. However, by arming yourself with this knowledge and making sure that appropriate steps have been taken to combat the effects of reactive soil in your home’s design and construction, you can at least breathe easy.

What you should do if you have reactive soil

Once you’ve established that your home is built on reactive soil, you should follow a few simple rules:

  • Keep the immediate surrounds of the house free from large trees or other significant vegetation. Trees suck moisture from the soil, causing the soil to shrink.
  • Ensure you maintain enough space between your home and its garden beds and irrigation systems so as to avoid unnecessary saturation of the soil around your house.
  • Ensure that your guttering and downpipes are in a good state of repair and adequate to direct rainwater away from the house into stormwater drains as opposed to into the soil itself.
  • Consider a comprehensive property inspection to detect any issues with your plumbing, sewerage and drainage systems and to uncover other problems caused by reactive soil.
  • Identify any damage to your property already caused by reactive soil and take steps to carry out repairs and preventative works before any existing problems worsen.

If you’re concerned about reactive soil and its impact on your property contact Action Property Inspections today on 1800 642 465.


About Andrew Mackintosh

Andrew Mackintosh has been answering home inspection questions in the greater Brisbane area since 1995. He has personally carried out over 20,000 building inspections and is a licensed builder and licensed building inspector, Member of Queensland Master Builders Association & Institute of Building Consultants. Being the business owner and the inspector, Andrew is passionate about providing clients with excellent customer service, value for money, honest, unbiased, thorough inspection and reporting in a professional and timely manner to allow clients to make informed decisions when purchasing property.